The present invention belongs to a technical field of thermal printers. More specifically, the present invention relates to a thermal printer capable of preventing dirt from adhering to specified positions of a thermal head.
Thermal printers have been conventionally employed to record small images such as those for ultrasonography on a thermal recording material (hereinafter referred to simply as a recording material) having a thermal recording layer formed on a support made of a film or the like.
The thermal printers have advantages such as the unnecessity of wet processing and simple handling, and hence have been also employed recently to record images for the purpose of diagnoses requiring high-quality large images, as exemplified by CT (Computerized Tomography), MRI (Magnetic Resonance Imaging) and radiography.
The thermal printers each use a thermal head having heating elements arranged in a line to thereby perform thermal recording. More specifically, the thermal head and a recording material are moved relative to each other in a direction perpendicular to a main scanning direction in which the heating elements are arranged, with the heating elements of the thermal head slightly pressed against the recording material, and energy is applied to each of the heating elements in accordance with an image to be recorded to heat a thermal recording layer of the recording material imagewise to thereby perform the thermal recording.
The recording material for use in the thermal recording is usually in the form of cut sheets with a predetermined size which are obtained by cutting. There are many cases where the thermal recording layer is exposed or warped in the lateral edge surfaces (edges in the main scanning direction) of cut sheets owing to the cutting of the recording material.
Therefore, the thermal recording very often causes the thermal recording layer exposed in the lateral edge surfaces of the cut sheets of the recording material to directly come into contact with the thermal head, whereby dirt is most likely to adhere to the thermal head. In general, the thermal printers perform the thermal recording by using a recording material of a single size or recording materials of several predetermined sizes. As a result, every time the thermal recording is repeatedly performed, the exposed thermal recording layer comes into contact with a specified portion of the thermal head and dirt is deposited onto the specified portion of the thermal head, which causes a decrease in the heat transfer to the heating elements positioned in the specified portion, resulting in a reduction of the color optical density on these heating elements.
As a result of the thermal recording performed in such a condition, portions recorded by the heating elements have reduced color optical densities and are made unclear, so that a finished image has streaky density unevenness.
Compared with thermal printers capable of printing on a recording material of a single size, in thermal printers capable of printing on recording materials of two or more sizes, the positions at which the lateral edges of the recording material come into contact with the thermal head particularly vary depending on the size of the recording material used.
In thermal printers capable of printing on recording materials of two or more sizes, dirt adheres to several portions on the thermal head depending on the recording material used. Therefore, in the case where the recording is performed with a large-sized recording material, there is a possibility that streaky density unevenness occurs not in the end portions of an image but in its important portions, which will pose a serious problem.
JP 2002-292953 A discloses a thermal printer which has a moving means for moving a thermal head and a recording material relative to each other in a main scanning direction at least by a length corresponding to one pixel, namely a recording material guide for shifting the transport path of the recording material in the main scanning direction, and in which the position at which the recording material comes into contact with the thermal head is changed with the moving means every time printing is carried out. This printer enables the frequency of contact between the lateral edges of cut sheets of the recording material and a specified portion of the thermal head to be reduced, and thus prevents concentrated adhesion of dirt to the specified portion of the thermal head.